This invention relates in one embodiment to the tapping of steel from a steelmaking furnace, and more particularly to a method for advancing the furnace angle during tapping.
1. Field of the Invention
A method and apparatus for tapping of steel from a steelmaking furnace such as an Electric Arc Furnace (EAF) into a receiving vessel such as a ladle. The method and apparatus are directed to advancing the furnace angle during tapping to minimize the disturbance of liquids in the furnace and to minimize the amount of slag entering the ladle during tap.
2. Description of Related Art
In a primary steelmaking operation, the steel is made in the presence of a primary steelmaking slag, mostly composed of oxides of metals, including calcium, silicon, iron, magnesium, aluminum and manganese. The primary slag is “oxidizing” with respect to acceptable steel oxygen contents prior to casting, and therefore it is not suitable for further steel refining operations. It is preferable not to let this slag flow from the furnace into the ladle, since it is harmful in the next step of steel refining which is “reducing”. Despite all efforts, some slag from the furnace always enters the ladle during tapping of steel. Since both the slag and the steel are molten, i.e. liquid phase materials, it is very difficult to prevent the entrainment of at least some slag in the flowing steel over the entire range of conditions that occur during tapping as the steel flows out of the tapping port.
As known in the art, attempts are usually made to eliminate primary steelmaking slag from ladle refining processes, either by retention of the primary slag in the primary steelmaking furnace as much as possible, or by raking from the surface of the steel in the ladle, or both.
Historically, steel was commonly tapped from an electric arc furnace through a tapping spout in the side of the furnace. This method resulted in substantial quantities of slag pouring from the furnace along with the steel. In U.S. Pat. No. 4,592,067, the disclosure of which is incorporated herein by reference, the inventors claim an apparatus designed to tap a furnace substantially “slag free” by replacing the tapping spout with a bottom tap hole, eccentrically placed. This and other similar arrangements have become common in the art. The eccentric bottom tap hole, or “EBT” as it is known in the art, has reduced the average amount of slag carried into the ladle from the furnace during tapping. However, in practice it has not produced the claimed result of slag free tapping. There have been improvements on this design; for example in U.S. Pat. No. 6,596,221, the disclosure of which is also incorporated herein by reference, a more complicated tap hole arrangement is described. Nevertheless, primary steel making slag is still commonly entrained into the ladle in commercial steel making facilities.
The amount of slag entering the ladle from the furnace in practice is quite variable, depending on many factors. It is known in the art, for example, that tilting the furnace backward quickly in order to terminate the tap tends to limit somewhat the slag entrainment into the ladle that happens at the end of tap. It is also known in the art that an old tap hole that has worn out to a larger inside diameter tends to entrain more slag through it during tapping than does a new tap hole of smaller inside diameter. It is also known in the art that a slag of higher temperature or more fluid physical character will more readily be entrained in the steel tapping stream and thus more slag will be carried into the ladle. The skill level of the operators that control the furnace angle during tap is also a factor that can increase or decrease the amount of slag carried from the furnace to the ladle during the tapping process.
The degree of success of the described equipment and methods in limiting slag entrainment in the flowing tapped steel is variable and unpredictable. Primary slag of varying amounts remains on the steel in the ladle when secondary refining is commenced. This is undesirable in that reagents and processing time are required to convert the primary slag to an acceptable secondary refining slag, thereby increasing the processing cost of steel making.
Accordingly, embodiments of the present invention are provided that meet at least one or more of the following objects of the present invention.
It is the object of the present invention to provide a method of tilting the steel making furnace during tapping in a way that consistently minimizes the amount of slag carry over into the ladle for any given set of process parameters and conditions.
It is a further object of this invention to provide a method of tapping the furnace that decreases the time required to tap the steel from the furnace, thereby increasing the steelmaking production rate and decreasing steelmaking cost.
It is a further object of this invention to provide a method to guide the operator in the advancement of the furnace tapping angle that is normalized to the predicted total tapping time.
It is a further object of this invention to provide an apparatus for tilting the steel making furnace during tapping in a way that consistently minimizes the amount of slag carry over into the ladle for any given set of process parameters and conditions.